Apartment buzzer or alerting systems have existed for many years and serve to alert a resident of the Apartment building of the arrival of a visitor to the main lobby area. Existing alerting systems use the Public Switching Telephone Network (PSTN) in various ways to alert the resident to the arrival of a visitor. One example of such a system is often referred to simply as a PSTN Buzzer system. The PSTN system generally has a visitor interface unit located in the apartment building and is connected over a regular telephone line to a telephone switch located at an end office which serves the local area. The visitor interface unit also includes an auto dialler which takes a resident name, an apartment number or some other means for identifying a particular resident of the apartment building and translates it into a PSTN directory number (DN) associated with the telephone physically located in the resident's apartment. To contact a particular resident a visitor uses the visitor interface to identify a particular resident and the auto dialler proceeds to dial a corresponding PSTN DN associated with the resident. Where the resident answers the call the telephone switch connects the visitor with the resident over the PSTN. Residents may allow visitors entry to the building by keying in a predetermined Dual Tone Multi-Frequency (DTMF) sequence which is detected by the visitor interface unit and used to unlock the lobby door. However, where the resident is already using his or her telephone, the visitor generally receives a busy tone leaving the resident unaware that a visitor is waiting in the lobby. Although an inexpensive system to install mainly for reasons that internal apartment wiring is virtually non-existent, residents can not distinguish visitor calls from regular calls and often are not able to be made aware of waiting visitors if they happen to be using their telephone at the time the visitor calls from the lobby.
A further more elaborate apartment building alerting system which does not directly use the PSTN to alert residents to the arrival of visitors is also in existence today. Such systems are often referred to as direct wired buzzer systems. Direct wired systems generally include a Line Interface Board (LIB) which is located at the apartment building and which terminates all individual telephone lines from the PSTN associated with DNs of respective residents in the apartment building. The LIB also includes direct wired connections internal to the apartment building directly to a telephone in each apartment. In such a system any incoming call from the PSTN to any resident of the apartment building, regardless of who or where the originator is, must be routed through the LIB. All calls originating from resident telephones and destined for the PSTN also must be routed through the LIB. A direct connection is provided from the LIB to a lobby located visitor interface and in a similar manner to the PSTN Buzzer system described above the visitor interface in response to a keyed entry, translates a resident name or an apartment number to a number associated with the corresponding resident telephone. The LIB then alerts the corresponding resident telephone over the direct wired internal connections without the need to route the call over the PSTN. When the call is answered by the resident a connection is made through the LIB between the visitor and the resident. To facilitate distinguishing visitor calls from regular PSTN incoming calls it is common for LIBs to be capable of applying a ringing signal to the resident telephone which is distinct from normal PSTN ringing signals. With limited service logic in the LIB, the direct wired buzzer system is able to apply for example a call waiting signal to alert a resident of a visitor any time the resident is on the telephone at the time a visitor calls. Despite the advantages of the direct wired system, the added initial expense of having to purchase a Line Interface Board as well as providing connections between it and every apartment telephone can easily make this system in the order of six times more expensive than the simple PSTN buzzer system just described. Apart from the initial high purchase price of such a system, all maintenance, servicing and upgrading costs also have to be absorbed by the owners of the apartment building. It is well known that service provider networks are designed to be extremely reliable and hence a further concern to residents is that because all of their telephone lines must interface directly with the LIB, their regular telephone service now depends on the integrity of this piece of third party equipment.